We are studying structural and functional aspects of histone specificity, by three approaches. (a) Changes in histones during sea urchin and Drosophila embryogenesis are being examined by analytical and preparative non-ionic detergent electrophoresis, a new method of separating proteins on the basis of their affinity for non-ionic detergents under denaturing conditions, as well as by conventional methods of solvent fractionation and electrophoresis. Limited chemical and isotopic tracer studies will be used to determine the chemical nature of these changes. (b) In case of the most variable class, the Fl histones, partial sequence analysis of widely divergent members, selected both from different developmental stages and from widely different members, selected both from different developmental stages and from widely defferent organisms, will be carried out to distinguish variable regions, related to specific functions, from constant regions that serve the general functions of this histone class. (c) The histones of Drosophila melanogaster, including minor components and Fl subtypes resolvable by non-ionic detergent electrophoresis, will be purifiid and characterized for use in immunochemical probes of histone arrangement in giant chromosomes.